Problem: Solve for $z$ : $15 = z - 6$
Solution: Add $6$ to both sides: $ \begin{eqnarray} \\ \hphantom{15 {+ 6}} &\hphantom{=}& \hphantom{z} \\ 15 &=& z - 6 \\ \\ {+6} && {+6} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 15 {+ 6} &=& z \\ \end{eqnarray} $ Simplify. $ z = 21$